1. Field of the Invention
This invention relates to optical glass fibres and to methods of forming such fibres.
2. Description of the Prior Art
Optical fibres for use in communication systems are required to have a low level of absorption or attenuation of light signals transmitted through them, so as to keep to a minimum the number of signal repeaters required in a transmission line. By careful control of the manufacturing process, so as to eliminate impurities, it is possible to reduce the attenuation to a figure of 10 dB per kilometer or less, but it is then found that attenuation due to colour centres induced in the fibre while it is being drawn can become important with certain types of high-silica optical glass fibre. The formation of such colour centres when drawing fibres from vitreous silica was reported by P. Kaiser in J. Am. Optical Soc., 64, 475 (1974). The principal wavelength of the absorption band caused by the colour centres was 630 nm. Optical fibres are generally drawn from pre-forms, i.e. glass rods with an outer region or cladding of lower refractive index than the inner region or core. In drawing fibres from pre-forms which have been made of a high-silica glass composition by the method disclosed in U.K. Patent No. 1,527,436, in which the core glass is modified by the addition of an alkali metal oxide which increases its refractive index, we have found a marked optical absorption band in the fibres centred at a wavelength of 510 nm, which is not found in optical attenuation measurements on the pre-forms and which disappears when the fibres are heated to about 500.degree. C. This absorption maximum at 510 nm can be as high as 250 dB/Km and the absorption band can be of such width that the attenuation in the range of wavelengths normally used in communication systems employing optical fibres, namely 800 to 900 nm, can be up to 20 dB/Km.
On the other hand, the high-silica glass compositions formed by the technique known as chemical vapour deposition and certain other high-silica glass compositions which are also used for drawing optical fibres and which employ additives such as GeO.sub.2 and P.sub.2 O.sub.5 to raise the refractive index of the core glass, do not appear to suffer from drawing-induced colour centre formation.
Colour centre formation in glasses is known to occur also as a result of the action of ionising radiation on the glass and cerium oxide in amounts of 1 to 2% by weight has been incorporated in glasses used, for example, for face plates in cathode ray tubes in order to suppress the formation of radiation-induced colour centres.
It has further been suggested by B. D. Evans and G. H. Sigal, Jnr., in IEEE Transactions on Nuclear Science, Vol. NS-22, No. 6, December 1975, pages 2462 to 2467, that it should be possible to reduce the sensitivity of optical fibres to ionising radiation and to improve the speed of recovery of light transmission by such fibres on removal of the radiation, by using fibres of silicate-based glasses incorporating cerium, though the known cerium-doped silicate glass fibres referred to in their paper, incorporating 0.2% to 1.2% Ce, were shown to have fairly high intrinsic losses (attenuations). In the operating wavelength range of 800 to 900 nm, we have found that the attenuation due to incorporation of cerium at these levels exceeds the attenuation due to drawing-induced colour centres which would be found in the absence of cerium.
U.K. Patent No. 816,412 describes the use of a smaller proportion of cerium, in a mean proportion of less than 10.sup.-3, in a glass otherwise consisting of pure silica, in order that the glass should retain its initial transparency in the visible spectrum and be free of induced radioactivity after having been subjected to the action of electromagnetic radiations or elementary particles, the glass being intended, for example, for making ampoules for irradiated liquids, for transparent windows or for optical assemblies which are intended for examination of articles liable to emit electromagnetic radiations or elementary particles. This glass is said, however, to show an ultraviolet absorption band with a maximum at 3200 A (320 nm). For the purposes indicated in Specification No. 816,412, an overlap of this band into the visible spectrum would be unlikely to cause problems. In an optical fibre communication system, however, because the typical path length is a kilometer or more, the resulting attenuation could be significant.